Quarterly report pursuant to Section 13 or 15(d)

Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail)

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Equity - Summary of Authorized and Outstanding Equity (Parenthetical) (Detail) - $ / shares
 6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2006
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Class Of Stock [Line Items]          
Preferred stock, par value $ 0.01   $ 0.01    
Class A Common Stock          
Class Of Stock [Line Items]          
Common stock, par value $ 0.01   $ 0.01    
Common stock, voting rights 1 vote per share        
Common stockholders rights percentage 100.00%        
Restricted stock units granted   10,582      
Restricked stock units unvested 5,687        
Common stock, shares outstanding 8,107,101   8,074,197 8,028,000 7,990,000
Class B Common Stock          
Class Of Stock [Line Items]          
Common stock, par value $ 0.01   $ 0.01    
Common stock, voting rights 1 vote per share        
Restricted stock units granted 727,726        
Class of share exchangeable to another class One share of Class A common stock        
Common stock, shares outstanding 4,852,711   4,866,303 4,671,000 4,695,000